Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common inherited disorders in humans, affecting one in a thousand people in the United States. Patients develop multiple cysts in both kidneys. The cysts progressively result in the destruction of the normal kidney structure and eventually lead to end-stage renal failure in approximately 50% of the patients by the age of 60. There are currently no cures for the disease. ADPKD is caused primarily by mutations of the PKD1 gene, which encodes polycystin-1 (PC1) protein. Therefore understanding of the normal structure and function of PC1 will be critical for the development of effective therapies. We have previously discovered that PC1 is proteolytically cleaved at the G-protein coupled receptor proteolytic site (GPS) in virtually all cell types and tissues in vivo. This reaction, the GPS cleavage, generates a number of previously unrecognized PC1 products. Defective GPS cleavage of PC1 has been found in a subset of ADPKD patients. We hypothesized that GPS cleavage is essential for the complete function of PC1 in the kidney. We have recently demonstrated using a novel mouse model that GPS cleavage of PC1 is essential for proper structure and function of the distal nephron segments in the kidney, but is apparently not required for that of the proximal segments and for embryonic development. We now propose that the cleavage products of PC1 are critically required for the structural integrity of distal segments of the nephron in the kidney. This grant aims at understanding the mechanism by which GPS cleavage regulates the important function of PC1 in the kidney. We propose the studies in three complementary Specific Aims using a combination of biochemical, biophysical, cell-biological methods and animal models. Specific Aim 1 will analyze structure and function of PC1 products generated by GPS cleavage in an in vitro MDCK model system. This study will likely establish basic principles of GPS functioning. Specific Aim 2 then looks at the role of the PC1 cleavage products in primary cells and finally in mice. Specific Aim 3 examines the reason why the proximal segments do not require GPS cleavage. The proposed studies will likely yield important insights into the function of PC1 in normal and disease states of the kidney. PUBLIC HEALTH RELEVANCE Polycystin-1 is the protein that when defective causes a complicated kidney disease known as autosomal dominant polycystic kidney disease, which at this point has no cure. This study aims to understand Polycystin-1, the reasons it becomes defective, and the ways to prevent it.